R.Brachman and B.Smith provide an excellent state-of-the-art overview for the specialist in knowledge representation. while A. Barr and E. Feigenbaum give an up-to-date overview for the nonspecialist. We hope that this special issue will introduce this important area of artificial intelligence to a wider audience. The core of the 15 articles, contributed by a broad spectrum of researchers on various aspects of knowledge representation, show the importance,diversity ,and vigor of knowledge representation as a research activity. Our introduction provides some background and context to these articles by mapping out the basic approaches to knowledge representation that have developed over the years.